This disclosure relates to acidizing fluids for use in subterranean applications, and more specifically, to complexing-acidizing self-diverting treatment fluids and methods relating thereto for use in subterranean formations comprising carbonates.
Methods to enhance the productivity of hydrocarbon wells (e.g., oil wells) or geothermal wells include matrix acidizing, which involves creating flow paths in the formation matrix by differential dissolution of small portions of the formation, or removing (by dissolution) near-wellbore formation damage, with an acid. “Wormholes” (conductive channels) are often generated through the matrix as a result and allow for improved conductivity within the formation. Acidic-based fluids are useful for this purpose due to their ability to dissolve both formation minerals and contaminants (e.g., contaminants such as drilling fluid filter cake on the wellbore or that has penetrated into the formation introduced into the wellbore/formation during drilling or remedial operations).
Acidizing carbonate formations can be challenging due to the relatively rapid reactivity of the carbonate with the acid. For instance, acid treatments in carbonate formations are plagued by at least two complications: (1) radial penetration, and (2) fluid loss of the acidizing fluid into the formation. The first problem, radial penetration, results from the fast reaction of the acid with the formation matrix upon introduction of the fluid in the near well bore region. The radial acid penetration is often limited to just a few inches to a few feet, which is not optimal. Those portions of the formation that are more distal to the wellbore (as one moves radially outward from the wellbore) remain untouched by the acid because the acid will fully spend. In fact, due to such limited penetration, it is believed that acid matrix treatments are limited in carbonate formations to those treatments focused on removing near-wellbore flow restrictions. Yet low permeability at any point along the hydrocarbon flow paths can impede flow (hence production), which is undesirable. Fluid loss of the matrix acidizing fluid into smaller wormholes neighboring the near well bore region only serves to exacerbate this radial penetration problem. Consequently, to try to achieve maximum radial penetration, prodigious fluid volumes are often required, which can prove costly.
One effort to improve radial penetration involves controlling the fluid loss of the matrix acidizing fluid so as to attempt to extend the radial depth of the main channels. Fluid loss in acidizing fluids for carbonate formations is typically controlled by the addition of polymeric gelling agents to the acidizing fluid. These polymeric gelling agents modify the relative viscosity of the treatment fluid, therefore, reducing or eliminating fluid loss flow into the natural permeability within the formation. However, such gels may prove problematic in that they can block conductive channels thereby reducing the conductivity of the formation if they are not removed prior to production. Extra steps are usually necessitated to remove the gel, such as breaking the gel downhole by adding an additional chemical breaker component to the formation and allowing sufficient time to pass to allow that breaker to break the gel, to improve conductivity.
Another method used to combat fluid loss is the use of fluid loss control agents, also known as diverting agents, like sand, quartz, salts, polymeric particles, and degradable particulates such as polylactic acid or polylactide particulates, to bridge the pore throats in the formation. Such agents may present barriers to conductivity as well if they are not removed or degraded for production. Unfortunately, degradation of such particulates can take periods in excess of weeks and even months, especially at temperatures below about 250° F., which is undesirable because that time impacts resource extraction. Another method to combat fluid loss is to emulsify the acidizing fluid. Emulsification involves an additional external component, which adds costs and complexity to the acidizing operation.
Yet another method to combat fluid loss involves the use of viscoelastic surfactants (VES). A VES-containing acidizing fluid often relies on the increase in viscosity when the pH of the treatment fluid increases to a pH where these surfactants transition to a gelled state. This occurs upon reaction of an acid with a source of carbonate, whether in the formation matrix itself or within a proppant pack, and the subsequent release of calcium into the fluid. These types of fluids can present the same sorts of problematic blockages and downtime mentioned above.